Angry flood survivors in Pakistan blocked a highway Monday to protest the government's slow delivery of aid amid the worst floods in that country's recorded history.

The floods began in northwest Pakistan more than two weeks ago and have spread throughout the country. Government estimates suggest as many as 20 million people and 160,000 square kilometres of land — about 20 per cent of the country — have been affected.

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Near the hard-hit Sukkur area, hundreds of victims used stones and garbage to block a major highway, complaining they were being treated like animals. Many said government officials were only handing out food when media were present.

"They are throwing packets of food to us like we are dogs," said protester Kalu Mangiani. "They are making people fight for these packets."

Coming days are 'crucial'

The flood's worst devastation is up north, in the Swat Valley, according to CBC reporter Adrienne Arsenault, who arrived in Pakistan last week.

Three weeks ago, the area would have been thriving, with busy market stalls, stores and hotels. Now, Arsenault said, they're gone — replaced by boulders and massive pieces of rock knocked from nearby cliffs.

"It really does look as if a giant has gone through the river valley, taking swipes at the landscape, taking out giant chunks and just chucking them down the river," Arsenault said.

"It is only when you meet people who say, 'I used to live right where that empty space is,' or 'My child's school was there' [that] you realize what on earth these people have lost."

More than two dozen bridges along the Swat River have been destroyed and many roads are impassable to vehicles.

Rescue crews are relying on donkeys to transport aid into remote regions, while villagers are left to carry aid bundles of mosquito nets and hygiene kits over hefty mountain passes by hand.

Fears remain that the dam in Sukkur, in northern Pakistan, might yet burst and cause more deadly damage.

It faced a major test of its strength as flood waters coursed down the Indus River into Pakistan's highly populated agricultural heartland, Sindh Irrigation Minister Jam Saifullah Dharejo said.

"The coming four to five days are still crucial," he said.

Calls for aid

UN Secretary General Ban Ki-moon said Sunday the flooding in Pakistan is the worst disaster he has ever seen, and he urged the world to do more for the flood-ravaged country.

"I will never forget the destruction and suffering I have witnessed today," he said after flying over some of the worst-hit areas. "In the past, I have witnessed many natural disasters around the world, but nothing like this."

The UN chief also pleaded for countries to speed up their assistance to the Pakistani people.

"We're dealing with individuals, families, mothers and children who have lost what little they had," said Kevin McCort, president of CARE, from the Swat Valley. "We know we have enough in Canada to share."

The UN has issued a global appeal for $460 million US in immediate help. Canada announced Saturday it would give $33 million in humanitarian aid.

Once the floods recede, billions more will be needed for reconstruction and getting people back to work in the already-poor nation of 170 million people. The International Monetary Fund has warned that the floods could dent economic growth and fuel inflation.